Chemical Sensor Seeks Out Land Mines, IEDs

Photonics.comAug 2012
STORRS, Conn., Aug. 3, 2012 — A new chemical sensing system that unearths land mines and other buried explosives by signaling their presence to the naked eye without advanced scientific instrumentation may be the first of its kind, University of Connecticut engineers say.

An estimated 110 million active land mines are hidden underground in 64 countries around the world, according to the United Nations. It is estimated that as many as 25,000 people are maimed or killed by these explosive devices each year. The mines not only threaten people’s lives, but they can also paralyze communities by limiting the use of land for farming or roads.

Clearing mines is a slow, deliberate process that often involves specially trained dogs and metal detectors, but each method has its shortcomings. Dogs, considered the gold standard in detection, tire easily and often have difficulty differentiating in dense minefields. Metal detectors are prone to false positive readings that can be triggered by buried pieces of metal unrelated to mines or unexploded ordnance.

University of Connecticut scientists have developed a novel buried explosive detection system using a nanofibrous film and UV light. (Top) A test application of the sensing film with trace 2,4-DNT explosive buried in a flowerpot. (Bottom) The same film under UV light 30 min later with the location of a buried explosive marked by a dark blot on the fluorescent film. (Images: Ying Wang/UConn and Advanced Functional Materials, Wiley-VCH Verlag GmbH & Co. KGaA)
Although explosive material can be concealed within land mines and IEDs, their seals often are not airtight, and small amounts of vapors escape, permitting detection.

The UConn sensor, made of nanofibrous film, can detect vapors from buried explosive devices with a chemical reaction that is visible when the film is exposed to handheld UV light. Examples include the nitroaromatics found in TNT and 2,4-DNT, and the elements used in harder-to-detect plastic explosives such as HMX, RDX, Tetryl and PETN.

If no explosive vapor is present, the recyclable film retains a bright fluorescent blue color when exposed to UV light, but if explosive molecules are present, the fluorescence is quenched, and a dark circle identifying the threat forms on the film within minutes.

"Our initial results have been very promising," said Dr. Ying Wang, who developed the system as a chemical engineering doctoral student under associate engineering professor Yu Lei. "We are now in the process of arranging a large-scale field test in Sweden."

(Top) A petri dish (left) with buried trace levels of 2,4-DNT explosive in soil and a petri dish (right) without DNT. (Bottom) Each petri dish after application of the chemical sensing film and after 30-min exposure under ultraviolet light. The location of the buried DNT appears in the petri dish (left) as a dark blot on the film.
Instead of using costly synthetic polymers or sophisticated chemical modifications to prepare their sensing material, the engineers prepared an ultrathin film, similar to paper, by electrospinning pyrene with polystyrene in the presence of an organic salt. This resulted in a highly porous nanofibrous membrane that can be rolled out over a suspect area like a sheet; it absorbs explosive vapors at ultratrace levels quickly and reliably.

The film also demonstrated sensitivity against common interferences such as ammonium nitrate and inorganic nitrates. Initial vapor detection took place within seconds with more than 90 percent fluorescent quenching efficiency within 6 min.

Wang and Lei have also developed a novel chemical test that detects TNT in water and other liquids. The application could be used to identify potential terrorist threats in airports as well as groundwater contamination in areas where explosives were used in construction. The ultrasensitive real-time sensor can detect TNT concentrations ranging from about 33 parts per trillion (the equivalent of one drop in 20 Olympic-sized swimming pools) to 225 parts per million.

The detection of particulate explosives on a contaminated hand using the novel electrospun pyrene film.
"Our new sensor based on a recently developed fluorescent polymer for explosives in aqueous samples has two sensing mechanisms in one sensing material, which is very unique," Lei said. "The sensor can easily be incorporated into a paper test strip similar to those used for pregnancy tests, which means it can be produced and used at a very low cost."

Wang and Lei have applied for patents for both chemical sensing systems.

The research was first reported in the May 11, 2012, online edition of Advanced Functional Materials.

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